1. Field of the Invention
The invention relates to a powdery mold-releasing lubricant and a mold casting method, and particularly intends to advantageously improve mold lubricity by effectively combining a powdery organic material which is decomposed or evaporated by heating with a powdery inorganic material.
2. Description of the Related Art
A powdery inorganic material having superior heat insulation and heat retention, such as talc, is used as a powdery mold-releasing lubricant in mold casting processes to reduce a flow rate of heat from a molten metal to a mold. However, in recent years, it is desired to develop such a mold-releasing lubricant that uses an inexpensive powdery inorganic material which does not necessarily have high heat retention to reduce a manufacturing cost of the mold-releasing lubricant.
That is, conventional powdery mold-releasing lubricants have utilized insulating properties of inorganic materials for heat retention of the molten metal, but selection latitude of the inorganic materials available for mold casting have been limited because of a limitation of powdery inorganic materials with good heat-insulation properties. For example, graphite is an inexpensive material and has good solid lubricity. However, since graphite is an electric conductor, its heat conduction caused by motions of free electrons is extremely high as compared to inorganic materials such as oxides, which posses a problem upon heat-insulating property, therefore, graphite cannot be used for such applications that require of heat insulation or heat retention.
As a solution for the above problem, it is considered to use a gas generated by decomposition or evaporation of a material consisting a powder as a heat-insulating boundary layer between the mold and the molten metal instead of utilizing the heat-insulation property of the inorganic material itself. However, it is practically impossible to form a thin heat-insulating boundary layer without any discontinuity between the molten metal flowing in the casting process and the mold from the gas generated by decomposition or evaporation of the organic materials alone.
The present invention has been developed in due consideration of the above situations. An object of the present invention is to provide a powdery mold-releasing lubricant which is inexpensive and has a good mold-releasing lubricity as well as a mold casting method using such a powdery mold-releasing lubricant.
The present inventors have strenuously repeated the study to achieve the above object. As a result, it is found that the desired object can be advantageously achieved by combining a powdery organic material which is decomposed or evaporated by heating with a powdery inorganic material.
That is, it is found that by mixing a powdery inorganic material and a powdery organic material, a movement of a generated gas is restrained (pinned) with the powder of an inorganic compound, and, as a result, a uniformly thin heat-insulating boundary layer is stably formed without any discontinuity between a mold or a sleeve and a molten metal. Herein, the powdery inorganic material is intended to pin the gas generated by evaporation or decomposition of the powdery organic material to form uniformly thin heat-insulating layer, not to secure the heat-insulating property as in the conventional lubricant.
Although a variety of methods for improving lubricity by mixing powders having different properties have been proposed, among such solid lubricants, there is no example that actively use the generated gas to improve the lubricity.
The gist and the constitution of the invention are as followed. 1) A powdery mold-releasing lubricant for use in casting with a mold, comprising a powdery mixture of a powdery organic material which is evaporated or decomposed by heating to generate a gas and a powdery inorganic material.
2) In the above 1), the powdery inorganic material is a powder of an inorganic material having a solid lubricity, said inorganic material being selected from the group consisting of graphite, kaolinite, SHIRASU (pumice stone) balloons, mica, zirconium silicate, carbon nanotube, carbon isotopes, talc, pyrophylite, crystalline SiO2, magnesium oxide, zirconium silicate, perlite and vermiculite.
3) In the above 1), the powdery inorganic material is a powder of an inorganic material having a solid lubricity, said inorganic material being selected from the group consisting of graphite, kaolinite, SHIRASU(pumice stone) balloons, mica, and zirconium silicate.
4) In the above 1) to 3), a mixed rate of the powdery organic material in the mixture is such an amount that can generate 10-50 ml of a gas per 1 g of the mixture.
5) In the above 1) to 4), an average particle size of the powdery inorganic material in the mixture is 1-30 xcexcm.
6) In the above 1) to 5), the powdery organic material is selected from the group consisting of polyethylene wax, metal soap, paraffin carbon hydride, sulfonic acid and sulfonic acid salt.
7) A mold casting method, comprising the steps of applying a powdery mold-releasing lubricant in the above 1) to 6) onto internal surfaces of a molding cavity and/or an injection sleeve, and feeding a molten metal into the molding cavity and/or the injection sleeve, whereby gas is generated from the mixture upon contacting between the fed molten metal and the lubricant, a gas-solid mixed layer of the generated gas and the powdery inorganic material is used as a heat-insulating boundary layer.
8) In the above 7), an amount of the powdery mold-releasing lubricant applied on the internal surfaces of the molding cavity and/or the injection sleeve is 0.01-10 g per 1 m2 unit area of.